Gliederung

The recording of electrically evoked compound action potentials (ECAP) of the auditory nerve via bidirectional telemetry using the intra-cochlear electrodes of the cochlear implant is a well established method. Cochlear as well as Advanced Bionics and Med-El provide hardware enabling this telemetric measurements in their present implant systems. The clinical use of this feature is company dependent, as the software development is on a fairly different stage at the moment.

The manufacturers build up a Babylonian confusion of tongues, probably because of marketing reasons. Neural response telemetry (NRT), neural response imaging (NRI) and auditory response telemetry (ART) are different terms for the telemetric ECAP recording.

The challenge of this method is to separate the response of the nerve from the stimulation artefact. Different algorithms have been introduced like forward masking techniques, alternating stimulation polarity, artefact template subtraction and 3rd phase adjustments.

Especially the work of DillierÂ´s group in ZÃ¼rich helped a lot to make the telemetric recording of ECAP a valuable clinical tool. Data are available on reproducibility and long-term stability of the ECAP amplitude and threshold. By this, the telemetrically recorded ECAP can be used as a diagnostic tool to monitor changes in the neural periphery.

Intra-operative recordings of the ECAP threshold can be done for every electrode without prolonging the time spent in the operating theatre significantly. On average the difference between intra-operative data and their first corresponding post-operative results agree well with postoperative test - retest variability. This makes it possible to use intra-operative ECAP data to assist the initial speech processor fitting. At the time being it does not seem to be possible to calculate the variables necessary to program the speech processors directly by using ECAP parameters like threshold, slope of the amplitude growth function or refractorial behaviour. But some semi-automated methods have been described to estimate first maps settings using ECAP data.

With software developments, like AutoNRT, that make the telemetric recording of electrically evoked compound action potentials of the auditory nerve easier for a not so skilled user and the implementation of ECAP based algorithms in the speech processor programming software, the ECAP will be developed to an essential tool in the rehabilitation of deaf patients with a cochlear implant.